1. Field of the Invention
The present invention relates to a process and apparatus for producing a high purity magnesia fine powder. More particularly, the present invention relates to a process and apparatus for producing a high purity magnesia powder which exhibit an improved sintering property.
2. Description of the Prior Art
It is well known that high purity magnesia exhibits an excellent heat resistance, electrical insulating property, and transparency, and therefore, is highly useful as a magnesia-type new ceramic material, for example, in high-quality porcelain articles, electrical insulators, heat resistant transparent materials, and infrared transmitting materials, for a wide field of industries.
Generally, the magnesia-type new ceramic materials are produced by shaping and sintering a ceramic powder comprising, as a principal component, magnesia (magnesium oxide, MgO). In the sintering procedure, the smaller the size of particles of the magnesia, the higher the density of the resultant sintered body, and the higher the density of the sintered body, the higher the mechanical, thermal, and transparency of the material.
In the conventional process of producing the sintered magnesia body, magnesia powders produced by calcining magnesia hydroxide or basic magnesium carbonate, are used as raw materials. However, because magnesia has a high melting point of about 2800.degree. C., in order to produce a sintered body having a high density, usually it is necessary to carry out the sintering procedure at a high temperature of 1700.degree. C. or more while the magnesia powder is hot-pressed.
The above-mentioned conventional sintering process is disadvantageous in that the process cost is high. Therefore, in order to reduce the sintering temperature to a level of from 1400.degree. C. to 1600.degree. C., and to reduce the process cost, the sintering property of the conventional magnesia powders is enhanced by mixing the magnesia powders with an additive comprising lithium fluoride (LiF), magnesium fluoride (MgF.sub.2), or sodium fluoride (NaF), or by treating the magnesia powder with a treating agent, for example, benzene.
For example, Japanese Examined Patent Publication (Kokoku) No. 49-16246 discloses a process for producing a transparent sintered magnesia tube in which process magnesia powder mixed with lithium fluoride and boron oxide, magnesium borate or boric acid is used as a raw material.
Japanese Unexamined Patent Publication (Kokai) No. 50-153798 discloses a process for producing magnesia having an improved sintering property, in which process a magnesium compound mixed with a fluorine compound such as magnesium fluoride is calcined at a temperature of from 600.degree. C. to 1200.degree. C.
M. Banerjee and D. W. Budworth, Trans. Brit. Ceram. Soc., Vol., 71 (3), 51-53 (1972) disclose a process for the preparation of transparent magnesia bodies, in which process magnesia powder mixed with sodium fluoride was sintered at a temperature of from 1300.degree. C. to 1700.degree. C. However, the above-mentioned processes, in which an additive is added to magnesia powder, result in a sintered magnesia body having a decreased degree of purity of magnesia. The additive in the sintered magnesia body causes the characteristic properties of sintered body to be degraded and, therefore, the use of the resultant sintered body is restricted.
Japanese Examined Patent Publication (Kokai) No. 56-16108 discloses a process for producing a high density sintered magnesia body, in which process magnesia powder mixed with a liquid hydrocarbon such as benzene are treated at a temperature of from 300.degree. C. to 650.degree. C. in an oxygen atmosphere, before the sintering procedure.
O. Yamaguchi, H. Tonami, and K. Shimizu, Chem. Lett., Vol. 8, 799-802 (1970) disclose that magnesia powder having an improved sintering property is produced by calcining magnesium hydroxide which has been prepared from magnesium alkoxide.
However, the above-mentioned processes are disadvantageous in that the processes are undesirably complicated and are costly, and, therefore, the resultant magnesia powder is expensive.
Also, in the above-mentioned processes, the magnesia powder is prepared by calcining a magnesium salt or alkoxide. In the calcining procedure, the crystals of the magnesia grow and the grown magnesia crystals form agglomerates. Therefore, the resultant magnesia agglomerates should be mechanically pulverized before the sintering procedure. However, even if the mechanical pulverizing procedure is applied to the magnesia powder prepared from the magnesium salt or alkoxide, it is very difficult to make the size of the particles of the pulverized magnesia satisfactorily fine, and the resultant pulverized magnesia still contains a certain amount of the agglomerates. The magnesia particles having an undesirably large size and containing a certain amount of the agglomerates results in an unsatisfactorily decreased packing density of the magnesia powder. This phenomenon causes the production of a high density sintered magnesia body to be difficult and the characteristic properties thereof to be degraded.
It is also well known that magnesia powder can be produced by the combustion of metallic magnesium in an oxygen-containing atmosphere. This reaction can be utilized to produce magnesia powder by a gas phase method. For instance, Czechoslovakian Pat. No. 139,208 discloses a process and apparatus for producing high purity magnesia powder by oxidizing magnesium vapor with oxygen. In this process, a flow of an inert gas containing magnesium vapor is brought into contact with a flow of oxygen gas in countercurrent relationship to each other, at a temperature of 700.degree. C. The resultant magnesia powder particles have a size of 1 .mu.m or less. However, we have found that as long as the process of the Czechoslovakian patent is used, it is difficult to obtain very fine magnesia powder particles having a size of 0.1 .mu.m or less.